Experiment Overview The established Animal Study Protocol investigates the supraspinal endogenous opioid system response to an acute noxious stimulus. This project has been accomplished in collaboration with our partners at the National Institute of Biomedical Imagine and Bioengineering using Positron Emission Tomography and a radioactively labeled tracer specific to opioid receptors, 18F FDPN. Evidence in the literature suggests that the injection of formalin (an acute noxious stimulus) induces the supraspinal release of endogenous opioid peptides. It has been hypothesized that endogenous opioid peptides released as a result of the formalin pain stimulus modulate pain behaviors. Administration of the opioid antagonist naloxone, a drug which will displace molecules bound to opioid receptors, increased nociceptive behaviors during the formalin model of tonic pain. This study allows us to address the following questions. 1) In what brain regions is opioid receptor availability altered by a noxious stimulus? 2) Can we replicate in a rodent model the findings from human studies showing opioid receptor displacement during a pain stimulus? 3) Does the tracer binding correlate with pain-related behaviors within specific brain regions? In addition, we will use the data from this experiment to plan and design our future projects related to chronic pain and chronic pain interventions. We have now collected all the data, and a manuscript has been written and is currently under peer review in the journal eLife.